1. Field of the Invention
The present invention as disclosed herein relates generally to a fluid motor operated by a pressurized fluid, such as a hydraulically or pneumatically operated motor, in particular, a positive-displacement fluid motor wherein a rotating member is rotated by means of flows of a pressurized fluid to and from a plurality of mutually independent fluid chambers. More particularly, the invention is concerned with a structure and a method of stopping such a positive-displacement fluid motor at a predetermined angular position, and a fluid control circuit for stopping the motor.
2. Discussion of the Prior Art
As a fluid motor, for example, a hydraulic motor, having a function of stopping at a predetermined operating position, there is known an indexing motor.
For stopping the indexing motor at a specific angular position, or positioning the motor, it is known to use a mechanism which is a combination of a mechanical valve, and a positioning cam with deceleration curves. In this mechanism, a lever is connected to the spool of the mechanical valve, so that the lever is pivoted about an axis as the spool is moved. The lever is provided at its one end with a roller pin. On the other hand, the positioning cam is attached to the shaft of the indexing motor, such that the roller pin on the lever engages a positioning groove formed in the circumferential surface of the positioning cam. The positioning cam is formed with the deceleration curves continuous with the leading and trailing ends of the positioning grooves. While the indexing motor is normally rotated, the mechanical valve holds the roller pin out of engagement with the positioning groove. When the motor is commanded to stop or to be indexed, the mechanical valve is activated, causing the lever to be pivoted in response to a movement of the spool, and thereby forcing the roller pin against the circumferential surface of the positioning cam, whereby the roller pin follows the deceleration curve at the leading end of the positioning groove. The resulting movement of the lever will enable the spool to restrict the flow of the fluid into the motor, thereby causing the motor to be slowed down. After the motor is slowed down following the deceleration curve, the roller pin comes into engagement with the positioning groove, whereby the motor shaft is mechanically stopped and positioned.
There is also known a brake motor which is similar to the indexing arrangement in that the motor is stopped, but uses a structurally different stopping arrangement. The conventional brake motor employs a mechanical braking mechanism, namely, utilizes friction to stop the rotation of the hydraulic motor. The mechanism includes a brake disc which is adapted to be forced against the rotating member of the motor. When the fluid supply and discharge flows to and from the motor are stopped, the brake disc is pushed onto the rotating member by a suitable push means such as a cylinder, which is actuated substantially in synchronization of a command to stop the fluid flows.
However, the conventional fluid motors have the following problems that must be solved. Referring first to the indexing motor, the need of the mechanical valve, lever, roller pin, positioning cam, etc. leads to complicating the construction. Further, the mechanism is disadvantageous in life expectancy and maintenance, since many portions of the mechanism are subject to friction. Moreover, the mechanical stopping of the motor with the roller pin engaging the positioning groove causes a comparatively large shock, and therefore a relatively long deceleration or slowdown period is necessary to mitigate the shock. Furthermore, if the motor overshoots the predetermined angular position, the motor phase cannot be returned back to that angular position, and the motor must be further rotated to be stopped during the next revolution, with another engaging action of the roller pin with the positioning groove.
The brake motor is also complicated due to the need of the brake disc and the push means for stopping the motor. The brake disc inevitably wears, and must be replaced by new one. This increases the frequency of maintenance services. Also, it is difficult to activate the braking mechanism at the moment when the flows of the fluid to and from the motor are stopped. Generally, the timing adjustment is such that the braking mechanism is activated a short time before the hydraulic flows are stopped. In this arrangement, the hydraulic motor continues to produce a torque even after the brake is applied, and the wear of the brake disc is aggravated.